In adaptive communications systems like UMTS, EDGE, or the 5 GHz WLAN systems (IEEE 802.11a, HiperLAN2, and HisWANa), transmission parameters like transmit power, code rate, and/or modulation scheme are adapted to the instantaneous conditions of communications links and, in particular, to channel conditions in order to make efficient use of the available resources. One key ingredient in these adaptation schemes is the so-called channel quality information (CQI) or communications link quality measurement (LQM), respectively. CQI or LQM stand for an assessment of link parameters (e.g., a signal-to-noise ratio (SNR), multipath tap weights, etc.)
A specific problem of CQI is the error-rate prediction for signal transmissions via multi-state communications links, e.g. coded transmissions over multi-state channels. The expression multi-state means that during the transmission of a signal or portions thereof (e.g. one code word) using a specific modulation and coding scheme (MCS), the communications link or channel state is varying. In the case of the instantaneous SNR or signal-to-interference ratio (SIR), this can be formally expressed by:yk=√{square root over (SNRk)}·xk+zk, ,k=1, . . . , N,  1.1wherein SNRk denotes the communications link SNR (channel state), which is experienced at instance k, and N is the number of considered signal portions per transmitted signal (e.g. the number symbols per code word). Here, the mean power of signal portions (code symbols) xk and noise samples zk are assumed to be equal to one.
There are numerous examples where these multi-state communications links appear. Three frequent situations shall be briefly mentioned in the following:                Transmission over time-selective,        frequency-selective, and        space-selective channels.        
An example for the transmission over a time-selective communications link is the physical layer transmission of UMTS. In general, the 5 MHz bandwidth of UMTS also leads to a frequency-selective channel. The information is encoded in the time domain and organized such that the incoming information is grouped, coded, and transmitted in so-called transmission time intervals (TTIs) of variable lengths (e.g. 10, 20, 40 or 80 ms).
One TTI directly corresponds to one code word. The received TTI symbols or soft bits after Rake combination are taken as decoder input. The channel SNR, or equivalently the channel state, experienced by these symbols is in general varying over time. The degree of variation depends on the vehicular speed.
One example for the time selectivity of a UMTS channel is depicted in FIG. 1. This is one snapshot for a vehicular speed of 120 km/h at a carrier frequency of 2 GHz assuming flat fading. The circles mark the beginning of the so-called slots, the length of which is 10/15 ms. The SNR values corresponding to the depicted circles may constitute the channel states SNRk. It should be noted that the SNR values are normalized such that the mean value equals one (0 dB).
OFDM transmission is an example for transmission over a frequency-selective channel. IEEE802.11a can be mentioned as one of the various systems applying OFDM. In OFDM, the information is encoded and mapped onto frequency subcarriers. Usually, it is applied in scenarios with large delay spreads with respect to the inverse transmission bandwidth, which means to have a frequency-selective channel over the transmission bandwidth. Therefore, the received subcarrier symbols after demodulation have in general experienced different fading amplitudes. The degree of subcarrier fading depends on the delay spread and the tap weights of the instantaneous channel impulse response.
One example for the frequency selectivity of a IEEE802.11a channel is depicted in FIG. 2. This is one snapshot for an rms delay spread of 150 ns, which is the assumption for the channel model C used in the IEEE802.11a standardization process. The IEEE802.11a channel bandwidth is 20 MHz and the band is divided into 64 subchannels. The circles mark the location of the individual subcarriers. The SNR values per subcarrier typically constitute the channel states SNRk. Again, the SNR values are normalized such that the mean value equals one.
MIMO transmission schemes of the BLAST-type may be mentioned as one example for a space-selective channel. Thereby, the (potentially physically available) parallel MIMO channels are fed by parallel transmit streams or layers. These layers may be encoded in common by one code and spatially multiplexed afterwards. The transmission over the MIMO channel results in interfering layers. Assuming a frequency-flat channel and applying a linear MMSE detector to the received signal vector in order to suppress the spatial interference, the symbol streams at the detector output, which are fed as soft bits to the decoder, experience in general different channel states in form of the detector output SNRs. This can be seen as channel varying over the space or the layers. The degree of variation depends mainly on the spatial correlation of the MIMO channel and also on the multipath profile.
One example for the space selectivity of a MIMO channel is depicted in FIG. 3. This is a typical snapshot for a flat and uncorrelated 4×4 MIMO channel. The transmit antenna streams or layers are separated at the receiver by means of a linear MMSE detector. The SNR plotted in the figure is the SNR at detector output. The SNR values per layer typically constitute the channel states SNRk. The SNR values are normalized such that the mean value equals one.
In multi-state communications scenarios as those mentioned above there is a need for a method and a receiver that allow to accurately determine the quality of a wireless communications link.